JPH0790666A - Electrolytic cation-exchange membrane and electrolytic method using the membrane - Google Patents

Abstract

PURPOSE:To conduct electrolysis practically without deteriorating an ion- exchange membrane even if an electrolyte is used as such by forming a porous layer not having an ion exchange function or one side and transferring the neutral point into the porous layer or on the porous surface. CONSTITUTION:The neutral point is transferred and positioned outside the part having an ion-exchange group and functioning as the ion-exchange membrane, namely, on the anode compartment side, in electrolysis. Consequently, impurities, if any, are deposited on the region outside the membrane where the ion exchangeability is not hindered, and the membrane is not broken or deteriorated. Further, almost the entire part having ion exchangeability is held highly basic, and electrolysis is continued at a low voltage by reducing the electric resistance of the membrane.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cation exchange membrane for electrolysis used for separation of acids and alkalis by electrolysis and an electrolysis method using the cation exchange membrane. More specifically, it relates to calcium. Even if a solution containing impurities such as magnesium and magnesium is electrolyzed without purifying and removing the impurities, a predetermined electrolytic reaction is performed with high current efficiency with almost no precipitation of the impurities in the cation exchange membrane. The present invention relates to a cation exchange membrane and an electrolysis method using the same.

[0002]

2. Description of the Related Art The production of chlorine and caustic soda by salt electrolysis is widely known as the most economical process due to the development of the ion exchange membrane method and the process for obtaining high quality caustic soda and chlorine. At the same time as the development of the process itself, research and development of the ion exchange membrane itself used in the process are also carried out, and it has an extremely low resistance and can maintain a high current efficiency of 96% even in caustic soda of about 35%. Is provided.

On the other hand, although the ion exchange membrane has such high performance, when the electrolyte contains impurities such as calcium ions and magnesium ions, these impurities are deposited as hydroxide precipitates on the alkaline side. The permissible level of impurities in the current ion exchange membrane method is several to several tens of ppb, and the purification of the electrolytic solution needs to be performed extremely strictly. Further, in the usual ion exchange membrane electrolysis, there is a problem that the anode side becomes strongly acidic and the cathode side becomes strongly alkaline with the ion exchange membrane as a boundary, the electric resistance of the ion exchange membrane itself becomes extremely large, and the electrolysis voltage easily rises. .

In the salt electrolysis, caustic soda and chlorine are mixed in a ratio of 1:
Although it is produced at a molar ratio of 1, chlorine is not used for many purposes and is likely to cause environmental pollution. Therefore, for example, natural soda ash (sodium hydrogen carbonate) or waste mirabilite (sulfuric acid) that does not contain chlorine as a raw material for electrolysis is used instead of salt. Sodium) has been proposed and attempts have been made to produce caustic soda from these raw materials. For example, the production of caustic soda from natural soda ash is carried out by a calcination method or an electrolysis method, but the former calcination method has a low purity of the product caustic soda, is difficult to use for the chemical industry, and is required to improve the purity in order to improve the purity. It has the drawback of leading to a rise.

The latter electrolysis method is an electrolytic separation of salt into acid and alkali, so that high purity caustic soda can be obtained.
As described above, the impurity content of the electrolytic solution has a large adverse effect on the ion exchange membrane. The natural calcium and magnesium content of natural soda ash represented by trona is several tens to 100.
It is ppm, and when electrolysis is performed using an electrolytic solution in which this natural soda ash is dissolved in water, the ion exchange membrane may be clogged or the ion exchange membrane may be destroyed. If refining is carried out to avoid this and the content of impurities is reduced to a permissible value, there is a problem that the cost becomes too high and it becomes economically impractical. Therefore, it is required to realize electrolytic separation of salt into acid and alkali by improving electrolysis conditions and ion exchange membranes without purification.

[0006]

SUMMARY OF THE INVENTION The present invention has the problems of the conventional ion exchange membrane electrolysis including electrolytic separation of salts into acids and alkalis, that is, 10 to 100 ppm or more of calcium ion, magnesium ion and the like. It is an object of the present invention to provide a cation exchange membrane for electrolysis that can perform electrolysis without substantially deteriorating the ion exchange membrane even if the electrolytic solution containing impurities is used as it is, and an electrolysis method using the cation exchange membrane.

[0007]

The present invention relates to a sheet-like cation exchange membrane for electrolysis having an ion exchange group and having a neutral point inside, and a porous layer having no ion exchange function on one side thereof. A cation exchange membrane for electrolysis and an electrolysis method using the cation exchange membrane, characterized in that a layer is formed and the neutral point is transferred to the inside of the porous layer or to the surface of the porous layer.

The present invention will be described in detail below. As described above, when an electrolytic solution containing impurities such as calcium ions and magnesium ions in the range of several ppm to 100 ppm is used for electrolytic reaction such as electrolytic separation, the following problems occur.
That is, in the case of a normal cation exchange membrane, the side facing the cathode chamber is strong alkali and the side facing the anode chamber is acidic, but its strength is not as strong as that of the strong alkali on the cathode chamber side. Therefore, it is assumed that the acidity changes in the thickness direction of the cation exchange membrane and the neutral point is the surface of the cation exchange membrane on the side of the anode chamber or a portion slightly entering the cation exchange membrane.

Then, the alkali metal ions and the impurity ions in the anode chamber move in the ion exchange membrane toward the cathode by electrolysis. At this time, the alkali metal ions, such as sodium ions, do not precipitate even when the pH changes and move in the ion exchange membrane as they are to reach the cathode chamber to generate caustic soda and the like, but the calcium ions and magnesium ions are Since it precipitates as a hydroxide at pH = 9 to 12, it precipitates on the surface of the ion exchange membrane or in the ion exchange membrane. The hydroxide deposited on the surface of the ion-exchange membrane can be removed by pickling, etc., but the hydroxide deposited in the ion-exchange membrane may destroy the crosslinks of the ion-exchange membrane, and the ion can be removed by pickling. However, the performance may not be recovered and it may become unusable in a short period of time.

In order to avoid this problem, in the present invention,
The neutral point is moved so as to be located outside of a portion having an ion exchange group and functioning as an original ion exchange membrane, that is, located on the anode chamber side during electrolysis, and even if precipitation of the impurities occurs, the precipitation is caused to occur. Of the ion-exchange membrane from outside of the ion-exchange membrane, which does not cause damage to the ion-exchange membrane or deterioration of its performance, and it is highly basic to almost all of the ion-exchange membrane. It is intended that the electric resistance of the ion exchange membrane be lowered and the electrolysis be continued at a low voltage.

In order to shift the neutral point from the original position to the anode side, (1) the current efficiency is lowered to increase the so-called reverse migration (back migration) amount, (2) the anode side of the ion exchange membrane. Two types of methods are conceivable, in which a liquid pool is provided on the surface so as not to be diffused by the flow of the liquid, and the alkali hydroxide which has been transferred from the cathode chamber side is retained on the surface of the ion exchange membrane on the anode side. Of these, the method (1) has the disadvantage that energy consumption increases as much as the current efficiency decreases and large equipment is required, and the method (2) uses a special ion exchange membrane or a processed ion exchange membrane. Although necessary, it is possible to prevent alkali hydroxide from diffusing to the side of the anode chamber during use and to obtain a substantially high current efficiency.

From this point of view, in the present invention, a porous layer having a function of holding a caustic alkali solution reversely transferred from the cathode chamber side to one surface of the cation exchange membrane, that is, the surface to be located on the anode side during use, is provided. Form. Since the cation exchange membrane according to the present invention is rarely used in a state of being in contact with the electrode during electrolysis, it is not necessary to impart hydrophilicity for taking out bubbles generated and the function of retaining the above-mentioned caustic alkali. Only, for example, silicon oxide having a particle size of about 20 to 50 μm, which is represented by diatomaceous earth, is kneaded with a fluororesin dispersion or the like as a binder, and the kneaded product is preliminarily formed with glass beads or the like on the surface. Apply to the side of the anode of the cation exchange membrane activated by blasting, ion beam or physical vapor deposition, and apply hot press to 150-30
The porous layer can be formed on the surface of the cation exchange membrane by baking at a temperature of 0 ° C. for about 15 minutes.

Corrosion resistant oxides such as titanium oxide and zirconium oxide may be used in place of the silicon oxide,
Further, the porous layer can be formed not by baking the powder but by attaching a porous film. Eg 2
A porous layer may be formed by attaching a membrane filter having a mesh size of about 10 μm or a polypropylene non-woven fabric.
On the anode side surface of the cation exchange membrane forming this porous layer, hydroxide of impurities precipitates and increases electrical resistance,
It is desirable to perform periodic pickling to remove the deposit. The surface of the cation exchange membrane on the cathode chamber side is exposed to strong alkalinity. Therefore, it is preferable to use a cation exchange membrane having acid resistance and alkali resistance to the extent that it can withstand the above pickling and strong alkali resistance.

As a cation exchange membrane satisfying these conditions, there is a perfluorocarbon type ion exchange membrane,
Among them, it is preferable to use a perfluorocarbon sulfonic acid type film rather than a perfluorocarbon carboxylic acid type film which is sensitive to impurities having high current efficiency and low electric resistance. The sulfonic acid type membrane is resistant to impurities, the obtained caustic concentration is relatively low at about 20%, and the current efficiency at that time is relatively low at 85 to 90%. It is considered that it becomes easier to transfer the hydrogen to the outside of the cation exchange membrane on the anode side.

The cation exchange membrane having this porous layer is placed in an electrolytic cell so that the porous layer is located on the anode chamber side, and an alkali metal salt solution is supplied to the anode chamber side to carry out electrolysis. The caustic alkali produced in step 1 is reversely transferred in the cation exchange membrane to reach the porous layer and is retained therein. The caustic alkali in this porous layer hardly diffuses into the anode chamber due to the liquid flow in the anode chamber, and diffuses into the anode chamber only by natural diffusion, and caustic alkali that balances with it is supplied from the cathode chamber, whereby the neutral point. Are located in the porous layer and the entire cation exchange membrane is kept alkaline. Therefore, hydroxides of impurities such as calcium ions do not deposit in the porous layer or on the surface of the porous layer on the anode side and do not deposit in the original cation exchange membrane. The cation exchange membrane will not be damaged even if it is used for electrolysis without performing.

Further, as described above, since the entire cation exchange membrane is alkaline, the electric resistance becomes small, and the spontaneous diffusion amount of caustic alkali in the porous layer is small, so that the amount of caustic alkali transferred in reverse is large. Therefore, the current efficiency is further increased and an efficient electrolytic reaction can be performed.

[0017]

EXAMPLES Next, examples of the production of the cation exchange membrane according to the present invention and the electrolysis method using the cation exchange membrane will be described, but the examples do not limit the present invention.

[0018]

Example 1 As an ion exchange membrane before treatment, a Nafion # 324 perfluorocarbon sulfonic acid type cation exchange membrane manufactured by DuPont was used. The surface of the cation exchange membrane used as the anode chamber side was blasted with glass beads for roughening and activation. Silica powder having a particle size of 5 to 50 μm and having impurities removed by an acid was kneaded with 30% by weight of Teflon (registered trademark) dispersion. This kneaded product is applied to the surface of the cation exchange membrane with a spatula so as to have a uniform thickness, an aluminum foil is placed on the surface, and the pressure is 0.2 kg / cm ² , the temperature is 200 ° C., and the time is 200 ° C.
It was baked by hot pressing under the condition of 15 minutes to obtain a cation exchange membrane having a porous layer.

This ion-exchange membrane was incorporated in a two-chamber electrolytic cell so that the porous layer was on the anode chamber side. Further, a hydrogen gas depolarized anode is used as the anode of the electrolytic cell, and hydrogen gas generated at the cathode is supplied to the anode chamber side through a conduit to form an electrolytic separator for separating salts into acid and alkali. did.

250 g / liter of Glauber's salt aqueous solution and deionized water were supplied to the anode chamber and the cathode chamber of this apparatus, respectively, and electrolysis was carried out under the following conditions. That is, temperature 50 ℃, current density
30 A / dm ² , the distance between the ion exchange membrane and the anode was 3 mm, the distance between the ion exchange membrane and the cathode was 0 mm (contact), and the decomposition rate of the aqueous solution of Glauber's salt was 50%, that is, the anolyte was Glauber's salt (sodium sulfate): sulfuric acid = The catholyte was 1: 1 (molar ratio), and the catholyte was a 20% aqueous sodium hydroxide solution. Note that the sodium sulfate contained impurities corresponding to 60 ppm as calcium ions.

As shown in Table 1, the initial cell voltage is 2.1 V.
And the cathode current density was 89-91%. The anode chamber was washed with 10% dilute sulfuric acid every two weeks, and electrolysis was continued for 6 months. The average voltage increased by 100 mV, but no decrease in current efficiency was observed.

[0022]

Comparative Example 1 The same Nafion # 324 perfluorocarbon sulfonic acid type cation exchange membrane as in Example 1 was used as it was without surface treatment, and electrolysis was performed under the same conditions as in Example 1, and the results are shown in Table 1. Initial voltage is 1.9 to 2.0 V
Although the current efficiency was slightly lower than that of Example 1, the current efficiency was slightly low at 85 to 90% and was not stable. When electrolysis was performed for 6 months while pickling every 2 weeks in the same manner as in Example 1, 6
After a month, the voltage increased by 200 mV and 100 mV from Example 1.
Was observed, and the current efficiency was reduced to 75 to 80%, which was 10% or more lower than that of Example 1.

[0023]

Example 2 A Nafion # 415 perfluorocarbon sulfonic acid type cation exchange membrane manufactured by DuPont was used as the ion exchange membrane, and the ion exchange membrane was sufficiently swollen in a dilute aqueous solution of caustic soda. After that, a polypropylene net with an opening of 35 μm and a thickness of about 100 μm was applied to the surface of the net using a Teflon (trade name) dispersion as an adhesive under a pressure of 0.5 kg / cm ² , 110 ° C., and a hot pressing method for 15 minutes. Bonded by.

The thus prepared ion-exchange membrane having a net bonded to the surface was incorporated into an electrolytic cell having an oxygen generating electrode having an iridium oxide coating as an anode and a Raney nickel-based activated cathode as a cathode, and the same as in Example 1. Electrolysis was performed under electrolysis conditions. Table 1 shows the cell voltage and current efficiency at the initial stage and after 6 months. Although the decomposition rate was increased to 60% and the pH of the anode chamber was set to 1 or less, the cell voltage did not change.

[0025]

Comparative Example 2 The same ion exchange membrane as in Example 2 was used except that no polypropylene net was adhered to the surface, and electrolysis was performed under the same conditions. The results are shown in Table 1. When the above decomposition rate was increased to 60% and the pH of the anode chamber was set to 1 or less, the cell voltage increased to 3.7 V, and it was found that the ion exchange membrane was greatly affected. As you can see from Table 1,
Since the pickling was performed every two weeks, the change in the cell voltage was slight, but in Comparative Example 2, there was a decrease in the current efficiency of 3 to 5% after 6 months, and the deterioration of the ion exchange membrane was clearly observed. However, in Example 2, the ion exchange membrane was not deteriorated and the current efficiency was kept high.

[0026]

[Table 1]

As can be seen from Table 1, in both the embodiment and the comparative example, the change in the cell voltage is small, but in the comparative example, there is a decrease in the current efficiency of about 3 to 10% which is considered to be caused by the deterioration of the ion exchange membrane. On the other hand, in the example, the current efficiency was kept high.

[0028]

INDUSTRIAL APPLICABILITY The present invention is a sheet-shaped cation exchange membrane for electrolysis having an ion exchange group and having a neutral point inside, and forming a porous layer having no ion exchange function on one surface thereof, A cation exchange membrane for electrolysis, wherein the neutral point is transferred to the inside of the porous layer or to the surface of the porous layer, and an electrolysis method using the cation exchange membrane.

This cation exchange membrane deposits the hydroxide of the impurities generated when the electrolytic solution containing the impurities is electrolyzed in the porous layer or on the surface thereof, so that the portion that originally functions as the ion exchange membrane is damaged. Therefore, it is not necessary to purify the raw material electrolytic solution, which requires a great deal of cost. Further, the porous layer retains caustic alkali that is reversely transferred from the cathode chamber and suppresses excessive reverse transfer, so that current efficiency is improved and efficient electrolysis can be performed.

Further, since the neutral point exists in the porous layer, the original ion exchange membrane is entirely alkaline, the ion exchange groups are always placed in a constant atmosphere, and the electric resistance decreases.
Economical operation becomes possible, and even if the pH of the anode chamber is lowered to promote the reaction, almost no increase in voltage is observed.

Claims (2)

[Claims] 1. A sheet-shaped cation-exchange membrane for electrolysis having an ion-exchange group and a neutral point inside thereof, wherein a porous layer having no ion-exchange function is formed on one side thereof, and the neutral point is formed. A cation exchange membrane for electrolysis, wherein the cation exchange membrane is transferred to the inside or the surface of the porous layer. 2. A cation exchange membrane for electrolysis in which a porous layer having no ion exchange function is formed on one side thereof and a neutral point is transferred into the porous layer or on the surface of the porous layer. Installed in the electrolytic cell so as to be located on the side of the electrolytic cell, and the electrolytic cell is divided into an anode chamber and a cathode chamber, and an alkaline salt aqueous solution of an inorganic acid is supplied to the anode chamber side for electrolysis, and alkali hydroxide is generated on the cathode chamber side. An electrolysis method characterized by obtaining.